from 1900 AI fleets without any battleships to 1920 legacy fleets which are pretty out there.
How about the possibility to load in a fleet, for yourself and for AI nations?
Eg within the Data folder, allow modders to make new folders, eg IDesHistorical, or IDesHardMode.
Within that we can put historical designs, with their historical class names.
And also files called fleetGreatBritain1900, fleetGermany1920, fleetGermany1920Versailles and so on, in which we can define the starting fleets for the various starts and fleet sizes, based on the designs within that folder.
Class = Wittelsbach
Class = Prinz Heinrich
would result in 1 Wittelsbach class ship and one Prinz Heinrich class ship, simply using the Wittelsbach and Prinz Heinrich designs within the same folder.
If medium fleet size is chosen, the Wittelsbach class ship "Wettin" is added and so on.
Just truncate the "Select slot for new game" window, and put a new selection in there based on the "IDesXYZ" folders found within the "Data" folder. If you click one of those, you load with those AI fleets (and your own if you do not select "build legacy fleet"), while the default is the current behaviour (fleet generated on IDes designs).